Electrically charged sheets, ie., electrets, are particularly attractive as a medium for printing posters and the like. These sheets stick to vertical surfaces via an electrostatic attraction, and hence, the sheets may be affixed to walls, windows, and the like without the use of fasteners or adhesives.
Electrets have been used in filters for some time. In such applications, a fibrous web is charged either by a hot or cold charging process. Hot charging processes are disclosed in U.S. Pat. No. 4,215,682 to Kubik, et al., U.S. Pat. No. 4,904,174 to Moosmayer, et al., and U.S. Pat. No. 5,122,048 to Deeds. These processes operate on fibrous web materials by charging meltblown fibers with an electrostatic charge immediately after they are formed and then deposited in a web. Such fibrous webs are not suitable for high quality printing applications. Further, the cost of such webs detracts from their attractiveness as a paper substitute.
There are also several cold charging processes for the preparation of charged webs. For example, U.S. Pat. No. 4,375,718 to Wadsworth, et al., and U.S. Pat. No. 4,588,537 to Klaase, et al., describe processes for the corona charging of webs made from layers of materials with differing conductivities. U.S. Pat. No. 4,592,815 to Nakao describes placing a nonconductive web between the surface of a grounded metal electrode and a series of discharge electrodes. U.S. Pat. No. 5,686,050 to Wadsworth, et al. discloses a corona charging method that uses a single electrode and a curved conducting roller to charge a fibrous web.
The degree to which the electret film is charged is important in both filtration and printing applications. Hence, it is advantageous to use as high an electric field as possible when charging the material. The high electric fields lead to arcing between the electrodes. While fibrous webs for use in filtration systems can sustain a significant amount of arcing damage, films for printing purposes cannot. When an arc forms between the charging electrodes the film is pierced, forming a small hole or other blemish. These blemishes can render the film useless as a print medium for high quality printed posters.
The prior art corona charging processes attempt to control arcing by shaping the electric field near the charging electrodes to prevent field gradients that can cause arcing. However, such solutions are less than perfect. Changes in humidity, particulate material, or the surface conditions of the rollers can lead to arcing even when the solutions presented in the prior art are utilized.
Broadly, it is the object of the present invention to provide an improved method and apparatus for charging a sheet or web.
It is a further object of the present invention to provide an improved method and apparatus for charging a sheet or web in which arcing is more easily controlled.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.